1. Field of the Invention
This invention relates to a vibrating type screen for image projection, and more particularly to a projection screen of vibration type which is periodically vibrated in a plane including the projection surface thereof. The vibrating type screen referred to in this specification broadly reads on a focusing plate ( including a Fresnel lens type focusing plate ) employed in optical instruments and is not limited to an ordinary image projection screen for a projector.
2. Description of the Prior Art
In the projection screen for viewing an image projected thereon by a motion picture projector or a slide projector, the transmission type screen used in a microfilm reader or an enalarging projector, or other projection screen as a focusing plate used in a viewfinder or monitoring device of other optical instruments, the resolution of the image projected thereon is deteriorated by the granularity of the material contained therein for the purpose of diffusing light for enhansing the diffusion of the projection scrren and enlarging the visible angle of view. For instance, in the reflection type screen, a metal screen as of aluminium having high reflectance is processed to have a mat surface or fine glass particles are mixed in a binder to be applied on the surface of the screen to enhance the diffusion effect. In a transmission type screen, a diffusion material like calcium carbonate is mixed in a binder and applied on the screen to enhance the diffusion effect. Further, in the focusing plate, the surface of the glass plate is processed to provide a mat surface to enhance the diffusion effect.
In these projection screens, it is required to reduce the size of the diffusion particles or enhance the grade of the mat surface in order to raise the resolution. This will, on the other hand however, lower the diffusion effect and lower the brightness of the image in the marginal portion of the screen. When the diffusion effect is lowered, not only the marginal portion of the screen becomes dark, but also the brightness of the image on the screen viewed at an angle with respect to the normal direction markedly falls and the angle of view in which the screen can be viewed with high brightness is reduced.
Therefore, in the conventional projection screens the resolution has been sacrified for obtaining high diffusion effect.
In order to make the resolution higher with the high diffusion effect, it has been proposed to finely vibrate the screen in the plane including the surface thereof thereby averaging the granularity of the screen effecting enhancement of the resolution. This type of vibrating screens have, however, disadvantages as follows. For instance, in U.S. Pat. Nos. 3,640,602, 3,125,927 and 3,186,299, there is disclosed a vibrating screen which is vibrated as a whole in the plane including the surface of the screen. This type of screen is disadvantageous in that the whole device is vibrated due to the vibration of the screen or a driving mechanism for vibrating the screen, and the vibration of the device transmits outwardly and brings nuisance to the environment and emits noise, which is undesirable in practical use.
Further, in a rotating type screen in which the screen is rotated about an axis of revolution emitting no vibrating noise, there is a defect in that circular lines become conspicuous. These lines are concentric about the axis of revolution of the screen and are particularly conspicuous near around the axis of revolution where the angular velocity is low. Since the center of the screen is usually important from the viewpoint of observation of the image, this type of screen is improper for practical use.